Frequency modulation receiver tuning indicator



sept. 15,1942.

FREQUENCY MODULATIQN RECEIVER 'TUNING 111010111011' piled lom. ,25, 1940 l INVENTOR EFmZsr BY ATTORNEY Patented Sept. 15, 1942 FREQUENCY MODULATION RECEIVER TUN- ING INDICATOR Dudley E. Foster, South Orange, N.

Radio Corporation of America, a

Delaware J., assignor to corporation of Application October 25, 1940, Serial No. 362,705

(Cl. Z50-20) to provide an audible tuning indicator for a fre- 9 Claims.

My present invention relates to frequency modulated carrier wave receivers, and more particularly to a device for audibly indicating accurate tuning of a frequency modulation receiver.

'I'he frequency modulation broadcast range at the present time is in the 43 to 50 megacycle band. It has been found necessary in this band to utilize visual tuning indicator devices for informing the operator when a desired broadcasting station has been accurately tuned in. Visual indicators are necessary because experience demonstrates that when the receiver is even slightly mistuned from the desired carrier frequency grave distortion occurs. This distortion arises by virtue of theA fact that a substantially S-shaped characteristic is utilized at the discriminator of the receiver, and the mid-point of the characteristic is the carrier frequency of the desired station. Tuning the receiver by ear has been found difficult, because of the fact that the receiver is being operated in an ultrahigh frequency range. This is particularly true at low volume passages. However, it is highly desirable to indicate tuning of the receiver by listening to the output thereof, and yet be able accurately to adjust the tuning of the receiver.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide an audible tuning indicator device for a receiver of frequency modulated carrier waves, the device permitting accurate tuning of the signal selector circuits of the receiver to the carrier, or center, frequency of the received frequency modulated carrier waves.

Another important object of this invention is to provide in a receiver adaptedv to detect frequency-variable carrier waves, a device which is responsive to the detector output circuit in such a manner that reproduction of the detected signals is impaired unless the receiver is tuned eX- actly to the carrier frequency of the desired signals.

Another object of the invention is to provide in a frequency modulation receiver a device which is responsive to both the limiter network and the discriminator network in such a manner that transmission of signals through the audio network of the receiver is prevented unless the receiver is accurately tuned to the center frequency of received frequency modulated carrier.

waves.

Still other objects of the invention are to improve generally the simplicity and efficiency of tuning indicators for frequency, or phase, modulated carrier wave receivers-and more especially quency modulation receiver which is not only reliable in operation, but is economically assembled in the receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself,` however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a receiving system embodying the invention,

Fig. 2 shows the response characteristics existing at the input and output circuits of the limiter of a frequency modulation receiver. Referring now to Fig. 1, there is shown only those networks of a frequency modulation receiver which are essential to a proper understanding of this invention. Since, at the present time, the superheterodyne type of receiver is used to receive frequency modulated carrier waves in the 43 to 50 megacycle band, it will be assumed that the numeral l denotes the resonant output circuit of an intermediate frequency' amplier which feeds the resonant input circuit 2 of the limiter tube. Each of circuits l and 2 will, of course, be tuned to the operating intermediate frequency, and the latter may be chosen from a range of 2 to 20 megacycles. It is stated by way of illustration that a value of 2.175 megacycles may be utilized.

It is not believed necessary to explain the nature of the networks preceding the circuit 1,' since those skilled in the art are fully aware of the specific nature of the prior networks in a superheterodyne receiver. Generally speaking one or more tunable radio frequency amplifiers may be included between the signal collector and the signal input circuit of the first detector network. 'Ihe Various tunable circuits are each resonated to the carrier frequency of the desired station. The local oscillator has its tunable tank circuit adjusted through a frequency range such that at each setting of the usual uni-control tuning means the rst detector output circuit, which is tuned to the operating intermediate frequency value, develops frequency modulated carrier voltage whose center frequency is at the op-V erating intermediate frequency value.

Of course, the circuit I may be the tuned output circuit of the first detector tube, or it may be located in the plate circuit of an intermediate frequency amplifier tube. The limiter network follows the usual construction well known to those skilled in the art. It may employ a tube 3, for example a sharp cut-off pentode tube, whose signal input grid is connected to the grounded cathode through the coil of input circuit 2 in series with the resistor 4, the latter being shunted by condenser 5. 'Ihe network 4 5 has a very short time constant, the resistor 4 develops direct current voltage thereacross in response to grid current flow in the limiter tube. Since the tube operates with little, or no, bias, grid current flows in the grid circuit when signals are received. It is, also, pointed out that the resonance curve of the input circuit of limiter tube 3 is single-peaked. In Fig. 2 there is illustrated in a graphic manner, and by the dotted line, the response at the input circuit of the limiter tube.

The plate circuit of the limiter tube includes the resonant circuit 6 which is tuned to the operating intermediate frequency. It is not believed necessary to describe the function of the limiter in detail. The limiter tube has an inputoutput characteristic as shown by the curve directly above the limiter tube. For signals above a predetermined carrier amplitude there occurs no further increase in limiter output.

This prevents any carrier amplitude variation from appearing at the discriminator. Since the discriminator-detector must be a device capable of converting solely frequency variation of the carrier into corresponding audio modulation, it

is essential that there be no carrier amplitude variation at the discriminator. Passage of the signals through the various tuned networks up to the limiter, noise impulses and fading may introduce carrier amplitude variation. It is necessary, therefore, to utilize a limiter network prior to impression of the signals upon the discriminator. Fig. 2 shows the relatively broadened response characteristic which exists at the limiter output circuit. The gain of the limiter determines the relative amplitudes of the two curves in Fig. 2. The output response could readily be greater, for example, than the input response amplitude.

lThe discriminator-detector may be of any well known form. It can be of the type utilizing a pair of rectifiers having output loads in phase opposition, and a common input circuit tuned to the operating intermediate frequency as disclosed by S. W. Seeley in U. S. Patent No. 2,121,103 granted June 21, 1938. It may comprise, alternatively, the type of detector which Vutilizes a pair of rectifiers whose output loads are in polarity opposition, but whose resonant input circuits are oppositely mistuned by equal frequency values from the operating intermediate frequency value. Merely by way of illustration this latter type of detector is shown in Fig. 1.

The detection network comprises one diode rectifier 1 whose anode is connected to ground through a path which includes the coil of its resonant input circuit 8 and the resistor 9. The load resistor 9 is by-passed for intermediate frequency current. The second diode I9 has a load resistor arranged in series with resistor 9, resistor being lay-passed for intermediate frequency currents. The anode of diode ||J is connected to the low potential side of input circuit 8 through the coil of resonant input circuit l2.

The cathode end of resistor is established at ground potential, and hence the voltages developed across resistors 9 and are in polarity opposition.

Circuits 8 and |2 are resonated to frequencies which are located on different sides of the center frequency of the impressed frequency modulated waves. It is not believed necessaryto specify the particular mistuning to be employed. It is l believed sufficient for the purpose of this application to point out that the circuits 8 and I2 will be oppositely distuned by equal frequencies to a sufficient extent from the center frequency such that a substantial voltage is developed across either of load resistors 9 or when the center frequency shifts towards the frequencies of the respective input circuits 8 and I2. The discriminator characteristic which is depicted above the push-pull connected diodes graphically shows the relation between center frequency deviation and rectified voltage output of the detection network. As is well known to those skilled in the art the characteristic is an inclined S-shaped curve.

The upper and lower spaced peaks of the curve are the resonant frequencies of the tuned circuits 8 and |2. When the signal energy applied from circuit 6 has its center frequency equal to the mean frequency of circuits 8 and l2 then the uni-directional voltage output of the detector is zero. It is not believed necessary to describe in detail the functioning of the detector circuit, since those skilled in the art are fully aware of the fact that the uni-directional voltages across load resistors 9 and are in polarity opposition. By grounding the cathode end of resistor there is derived from the cathode end of resistor 9 rectified voltage which corresponds to the modulation signals applied to the carrier at the transmitter. In other words, the audio voltage taken ofi from the cathode end of resistor 9 corresponds to the frequency modulation of the transmitted carrier wave produced by the audio waves at the transmitter.

. In order to tune the receiving system by ear, and yet secure an accurate tuning adjustment, there is provided a relay device which comprises a magnetic armature 2. The latter is disposed adjacent a pair of high resistance windings 2| and 22. The winding 2| is connected between the grid end of resistor 4 and ground. The connection includes a filter network 23 for removing any alternating current voltage components. The pivoted end of armature 20 is connected by lead 24 to the cathode end of resistor 9. The winding 22 is connected between the cathode end of resistor 9 and ground through a path including lead 25 and the filter network 26. The latter filter is also used to suppress any alternating current components. Hence, it will be seen that there ilows through windings 22 and 2| solely direct currents.

A spring 30 is arranged to maintain the armature 20 in substantially mid-position between windings 2| and 22. In this position the contacts di) are normally open, or spaced apart. One of the contact elements is electrically connected to the armature 20, while the other contact element is connected by lead 50 to the input circuit of the following audio frequency amplifier network. The audio frequency amplifier may include one or more stages of audio amplification followed by an audio reproducer.

When no current ows through the windings 2| and 22 the armature 2|] assumes the position shown in Fig. 1, and the contacts 40 are in opencircuit position. In other words the transmission path, comprising 24-40-50, to the audio network is open, and the reproducer will not reproduce signals. Hence, in the absence of received signals, or in the presence of signals whose amplitude is too low to cause voltages from appearing across resistor 4 or across resistors 9 and there will be no reproduction of even noise impulses. `As the receiver is tuned to a desired station a certain amount of grid current flow is produced through resistor 4. However, the magnitude of the current is not a maximum by virtue of the response characteristic at the input circuit 2. From Fig. 2 it is seen that the response characteristic is rather sharp. Unless the receiver is accurately tuned to the center frequency of the frequency modulated waves,` the current flow through resistor 4 has a value less than the peak value at center frequency resonance.

While grid current flow through resistor 4 will energize the winding 2|, and, therefore, tend to pull the armature 20 towards it against the biasing action of spring 30, this action is opposed by the energization of winding 22. The winding 22 is energized so long as the mean, or center, frequency of the applied frequency modulated signals is different from the predetermined mean frequency of the resonant frequencies of circuits 8 and I2. The latter predetermined center frequency is the operating intermediate frequency. Unless the receiver is accurately tuned to the center frequency of the collected frequency modulated signals, the uni-directional current component of the rectified voltages appearing across resistors 9 and has a magnitude depending on the amount of frequency displacement between the predetermined intermediate frequency and the center frequency of the applied signals. Not only will this unidirectional current flow through winding 22I by virtue of. the frequency difference mentioned, but it Will be seen from the limiter output response characteristic in Fig. 2 that the effect of the limiter is to give an apparent broadening of the response curve which exists at the input circuit of the limiter. Hence, there will be a substantial energization of winding 22 as long as there is a frequency difference between the center frequency of the frequency modulated intermediate frequency energy and the predetermined intermediate frequency value. In general, winding 22' will have sufficient energization for any detuning near the signal to overcome the effect of winding 2|. This can be adjusted by limiter gain, detector sensitivity and relative effect of windings 2| and 22.

This energization of winding 22 is sufficient to pull the armature 26 away from winding 2|, and thus maintain the contacts 4U in spaced relation. In other words the audio transmission path is disabled. Hence, the operator knows that the receiver is not accurately tuned. However, as soon as the receiver is accurately adjusted to the point where the center frequency of the frequency modulated intermediate frequency energy is equal to the predetermined intermediate frequency value, then the unidirectional voltage developed across resistors 9 and is zero and coil 22 is de-energized. Simultaneously the grid cur-` rent flow through resistor 4 is a maximum, and armature 20 is attracted to winding 2| so as to close contacts 4|J. This results in transmission of the audio voltage between the detector output load and the audio amplifier ,input terminals;V

The operator now knows that he has accurately tuned the receiver, since he is able to develop audio output from the system. Should he Wish to tune to another station therewill be a rapid impairment of audio reproduction. Upon slight detuning the current fiow through resistor 4 drops, and the compressed thereby opening contacts 40. As explained before, winding 22 will be energized upon detuning and thereby aid in the immediate opening of contact elements 4|).

It will be appreciated that the magnitudes of.

coils 2| and 22 may be adjusted 'to provide vary` ing speeds of actuation of armature 2|) depending on the will of the operator. windings 2| and 22 there may be used current windings in series with the loads. Furthermore, the biasing spring 3U may be replaced by any other device. For examplel the relay armature 20 may be arranged to open on accurate tuning if desired. In such case the contacts 40 would be in shunt with the audio network. The contacts 40 could operate to change a normal cut-off bias on an audio amplifier tube. Again, in place of a mechanical relay an electronic device, such as an electron discharge tube, may be used to change a normal cut-off bias existing on an audio amplifier tube in the absence of accurate tuning. In such case audio frequency transmission will occur solely when the limiter tube is energized and the output voltage across the detector load resistors is substantially zero as explained previously.

If it be assumed that in the absence of signal a portion of the audio amplifier is biased to cut-off, winding 22 can be replaced by two tubes so arranged that if either is energized audio bias will be at cut-off, and in series therewith a third tube replacing winding 2| and arranged so that in the absence of signal (when both tubes replacing 2`2 would be de-energized allowing normal bias on the audio) it will place cut-off bias on the audio tube. The requirements for audio transmission will then only be met on correct tune when the two tubes replacing 22 have no net effect, and the tube replacing 2| has maximum effect.

Furthermore, it will be alfpreciated` that winding 2| may be replaced by a spring which normally pulls armature 28 into position to close contacts 40, and in such case the armature 20 would be pulled away in a direction to open contacts 4@ as long as winding 22 was energized by the uni-directional voltage output of the detector'. In general, then, it will be seen that there has been provided by this invention a device adapted audibly to indicate accuracy of tuning in a frequency modulation receiver by maintaining audio reproduction impaired except when the center frequency of the received frequency modulated carrier waves is exactly equal to the predetermined frequency corresponding to the desired station carrier frequency.

While I have indicated and described a system for carrying my invention into effect, it will4 be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a receiver of frequency modulated carrier Waves of the type provided with a frequency modulation detector and a utilization network spring 3U will immedi- `ately urge armature 20 away from winding 2`|l In place of voltagel for detected waves, the method which includesv deriving from the detected waves aunidilre'ctional voltage whose magnitude is a function deriving a second uni-directional voltage from thefrequency modulated carrier wavesapplied to said detector and regulating said transmission in an opposing sense with said secondl voltageV in response to the latter assuming a max-i mum'value.

2 In a receiver of frequency modulated' carrier waves of the type modulation detector and a utilization network for detected waves, the method which includes deriving from the detected waves a uni-directional vcltage whose magnitude is a function of the frequency difference between the center frequency of the frequency modulated carrier waves applied to said detector and a predetermined frequency value, regulating the transmission of said detected waves network with said uni-directional voltage, and automatically regulating said transmission in a sense opposed to said first regulation in response to maximization of the amplitude of the center frequencyv of the frequency modulated carrier waves applied to said detector.

3. In a receiving system of the type provided with a frequency modulation detector having an input circuit to which are applied frequency modulated carrier waves, and a-modulation voltage transmission network following the detector; theimprovement which includes means for derivingfr from the detector a direct currentvoltage which is a` function of frequency difference between the center frequency of the frequency modulated carrier waves at said input circuit and a predetermined frequency value of! said input circuit,

of modulationvoltage through said transmission network, means for deriving a uni-directional voltage from the waves at said input circuit, and'means responsive to solely a maximum value of said uni-directional voltage for opposing the effect of said impairing means.

4. In a receiving system of the type provided with a frequency modulation detector having an input' circuit to which are applied frequency modulatedcarrier waves, and a modulation voltage transmission network following the detector; the improvement which includes means for deriving from the detector a direct current voltage which is a function of frequency difference between the center frequency of the frequency modulated carrier waves at said input circuit anda predetermined frequency value of said input circuit, means responsive to said direct current voltage for impairing the transmission of modulation voltage through said transmission network, and means responsive to the amplitude of the center frequency of said modulated carrier waves at said input circuit for opposing the effect of lsaid impairing means.

5. In a receiving system of the type provided with a frequency modulation detector having an input circuit to which are applied frequency modulated carrier waves, and a modulation voltagetransmissionnetwork following the detector;

the improvement which includes means for de' provided with a frequency to said utilizationmeans responsive to said direct current voltage for. impairing the transmission rivingfrom the detector a dir-ectcurrent voltage which is afuricti'onJ of' frequency difference between the center frequency of the frequency modulated carrier waves at said input circuit and a predetermined' frequency value of said input circuit', and means. responsive to said' direct current Voltage 6. In asreceiver of frequency, or phase, modulated carri'er waves of theA type provided withv a detector and! a utilization network for detected waves, the' method whichv includes deriving from the detected waves a direct current voltage whose magnitude is a function of the difference between the center frequency of the modulated carrier waves applied to said detector and a predetermined frequency value, inhibiting the transmission of said detected waves to said utilization network with said voltage, deriving a second voltage from the modulated carrier waves applied to said detector, and controlling said transmission in an opposing'sense with said second voltage in response to the latter assuming solely a maximumV value.

7`. In a receiver offrequency, or phase, modulated carrier waves of the type provided with a detector and a utilization network for detected waves, the method'which includes deriving from the detected waves a direct current voltage whose magnitude isa function of the difference between the center frequency of the modulated carrier waves-applied to said detector and a predetermined frequency value, controlling the transmission of said detected waves to said utilization network with saidv voltage, and automatically controlling saidv transmission. in a sense opposed to said rst control in response to an increaseintheamplitude of the center frequency of thev modulated carrier waves applied to said detector.

8; In a receiving system of the type-providedV with. a frequency modulated detector having an input circuit to which are applied frequencyV modulated-carrier waves, and an audio voltage transmission networkv following/the detector; ther improvement which includes means for deriving from the detector adirect current voltage which is proportionalY in magnitude to the frequency difference between the center frequency of thev frequency modulated'carrier waves at said input circuit anda predetermined frequency value of said input circuit, meansv responsive to saiddirect` current Voltage for impairing the transmission of audio voltagev through said'transmis'- sion network, means for deriving a direct cur-4 rent voltage from the wavesf at said input circuit, and means responsivev to maximization of said latter-voltage for opposing the effecty of said` impairingv means.

9. In a receiver of angular'velocity-modulated carrier waves, a limiter tube having input-andput network coupled to said'output electrodes,f

for impairing the trans-y mission of modulationy voltage through said' said input network having resonance curve which is relatively broad, means adapted to apply the modulation Voltage output of said detector to a utilization circuit, said discriminatoredetector network being constructed and arranged to produce a uni-directional Voltage whose magnitude is a function of the difference between the midband frequency of applied Waves and a predetermined frequency, means responsive to production of said uni-directional voltage for impairing operation of said applying means, means for deriving a second uni-directional Voltage from said limiter input circuit which maximizes at said single peak frequency, and means responsive to the second voltage for opposing the action of said impairing means.

DUDLEY E. FOSTER. 

